Classifications

• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
  • C08L101/00—Compositions of unspecified macromolecular compounds
  • C08L101/12—Compositions of unspecified macromolecular compounds characterised by physical features, e.g. anisotropy, viscosity or electrical conductivity
  • C08L101/14—Compositions of unspecified macromolecular compounds characterised by physical features, e.g. anisotropy, viscosity or electrical conductivity the macromolecular compounds being water soluble or water swellable, e.g. aqueous gels
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61L—METHODS OR APPARATUS FOR STERILISING MATERIALS OR OBJECTS IN GENERAL; DISINFECTION, STERILISATION OR DEODORISATION OF AIR; CHEMICAL ASPECTS OF BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES; MATERIALS FOR BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES
  • A61L15/00—Chemical aspects of, or use of materials for, bandages, dressings or absorbent pads
  • A61L15/16—Bandages, dressings or absorbent pads for physiological fluids such as urine or blood, e.g. sanitary towels, tampons
  • A61L15/42—Use of materials characterised by their function or physical properties
  • A61L15/60—Liquid-swellable gel-forming materials, e.g. super-absorbents
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
  • C08F8/00—Chemical modification by after-treatment
  • C08F8/14—Esterification
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J3/00—Processes of treating or compounding macromolecular substances
  • C08J3/24—Crosslinking, e.g. vulcanising, of macromolecules
  • C08J3/245—Differential crosslinking of one polymer with one crosslinking type, e.g. surface crosslinking
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J2300/00—Characterised by the use of unspecified polymers
  • C08J2300/14—Water soluble or water swellable polymers, e.g. aqueous gels

Definitions

• the invention relates to pulverulent, crosslinked polymers which absorb water, aqueous liquids and blood (superabsorbers) and have improved properties, in particular an improved retention and an improved retention capacity for liquids under pressure and an improved capacity for transportation of liquids, their preparation and their use as absorbents in hygiene articles and in industrial fields.
• Superabsorbers are water-insoluble, crosslinked polymers which are capable of absorbing large amounts of aqueous liquids and body fluids, such as e.g. urine or blood, with swelling and the formation of hydrogels, and of retaining them under a certain pressure. As a result of these characteristic properties, these polymers are chiefly used for incorporation into sanitary articles, such as e.g. babies' nappies, incontinence products or sanitary towels.
• the superabsorbers which are currently commercially available are substantially crosslinked polyacrylic acids or crosslinked starch-acrylic acid graft polymers, in which some of the carboxyl groups are neutralized with sodium hydroxide solution or potassium hydroxide solution.
• permeability is understood as meaning the capacity for transportation and three-dimensional distribution of added liquids in the swollen state. This process proceeds in the swollen superabsorber gel by a capillary transportation through intermediate spaces between the gel particles. Transportation of liquid through swollen superabsorber particles itself follows the laws of diffusion and is a very slow process which plays no role in the distribution of the liquid in the use situation of the sanitary article. In superabsorber materials which cannot effect capillary transportation because of a lack of gel stability, separation of the particles from one another, avoiding the gel blocking phenomenon, has been ensured by embedding these materials into a fiber matrix.
• the degree of crosslinking of the polymer can be increased, which necessarily results in a reduction in the swellability and the retention capacity.
• An optimized combination of various crosslinking agents and comonomers, as described in the patent specification DE 196 46 484, is indeed capable of improving the permeability properties, but not to a level which allows, for example, incorporation of a layer which optionally comprises only superabsorbers in a nappy construction.
• Methods for surface after-crosslinking of the polymer particles can furthermore be used.
• after-crosslinking the carboxyl groups of the polymer molecules on the surface of the superabsorber particles are reacted with various after-crosslinking agents which can react with at least two of the carboxyl groups close to the surface.
• the ability to absorb liquid under pressure is greatly improved in particular, since the known phenomenon of gel blocking, in which swollen polymer particles stick together and as a result a further absorption of liquid is prevented, is suppressed.
• after-crosslinked polymers show a good absorption without the use of pressure.
• superabsorbent polymers which are already surface-crosslinked can be treated against free-radical degradation by body fluids, in particular L-ascorbic acid, by after-treatment with a compound of titanium or zirconium and a compound which chelates these metal compounds.
• EP 0 233 067 describes water-absorbing resins which are crosslinked on the surface and are obtained by reaction of a superabsorbent polymer powder with 1-40 wt. %, based on the polymer powder, of an aluminium compound.
• 100 parts by wt. of crosslinking agent solution are applied to 100 to 300 parts by weight of absorber.
• the diols e.g.
• polyethylene glycol 400 and 2000, 1 ,3-butanediol or 1,5-pentanediol) added to the reaction medium of water also serve to prevent lumping together of the superabsorber during treatment with the large amounts of aqueous treatment solution used here.
• the solvent is removed in a subsequent drying step at 100° C.
• the polymers treated in this way have an inadequate level of properties, and an improvement in the absorptive ability under pressure is not achieved. Furthermore, treatment with large amounts of treatment solution cannot be carried out economically in modem, continuously operating processes.
• WO 96/05234 describes a process for the treatment of superabsorbent polymers, according to which the surface of the absorber particles, which contain at least 10 wt. % water, was treated with a crosslinked layer obtained by a reaction of a reactive, hydrophilic polymer or a reactive organometallic compound with an at least bifunctional crosslinking agent at temperatures below 100° C.
• Metal salts are not mentioned.
• the metal compounds employed must be able to react with the functional groups of the crosslinking agent. Organometallic compounds are therefore recommended as the metal compounds, and should be present in a weight ratio of 0.1 to 30 to the crosslinking compound.
• the polymers obtained are said to have a balanced ratio of absorption, gel strength and permeability, the measurement values stated being determined under less critical conditions. Thus, for example, the absorption and the permeability are determined without any pressure loading.
• a disadvantage of this known process is the use of solvents and toxically unacceptable crosslinking reagents, such as e.g. the polyimines, alkoxylated silane or titanium compounds and epoxides mentioned as preferred.
• the Japanese laid-open specification JP-A-09124879 is directed to after-crosslinking of the surface with polyfunctional crosslinking agents, the water content of the polymer particles again being adjusted to 3-9 wt. % after the surface-crosslinking and it being possible for this amount of water to contain inorganic compounds, such as metal salts.
• Superabsorbent polymers which, according to WO 98/48857, are brought into contact in particle form with polyvalent metal salts by dry mixing and are then provided with a certain amount of a liquid binder, such as water or polyols, are said to have an improved gel blocking during absorption of aqueous liquids.
• the polymer particles can be subjected to after-crosslinking of the surface before this treatment.
• WO 98/49221 recommends re-moistening of the polymer particles to the extent of up to 10 wt. % water with an aqueous additive solution.
• aqueous solutions can contain mono- or polyvalent ions or propoxylated polyols. It is also possible for the polymer particles already to be brought into contact with the aqueous additive solution before the after-treatment of the surface, as a result of which a more uniform distribution of the agent for after-treatment of the surface is said to be achieved.
• the object of the present invention is to provide superabsorbent polymers which have an improved combination of properties, in particular not only a high absorptive ability under pressure, but also combine the conventionally opposing properties of a high retention capacity and a good permeability, i.e. have a level of the combination of properties at which, in addition to a retention value of at least about 25 g/g, at least an SFC value of at least, 45 ⁇ 10 ⁇ 7 , preferably at least 50 ⁇ 10 ⁇ 7 cm 3 sec/g is present.
• an object is to provide superabsorbent polymers which are suitable above all for use in very thin nappy constructions with a very high superabsorber content. For this case, polymers with retention values of at least about 25 g/g and permeability values of SFC more than 70 ⁇ 10 ⁇ 7 cm 3 s/g are required in particular.
• a further object of the invention was to discover preparation processes for such superabsorbent polymers which can be carried out simply, economically and reliably, give a uniform product quality and in which in particular small amounts of solvent are used and where possible organic solvents are avoided. It should furthermore be possible to carry out the processes without the use of toxicologically unacceptable substances.
• % based on the polymer, excluding crosslinked partly neutralized polyacrylic acid which has been treated with Al 2 (SO 4 ) 3 and glycerol in a weight ratio of 1:1 or with Al 2 (SO 4 ) 3 ⁇ 16H 2 O and polyethylene glycol in a weight ratio of 1:1.8 or with Al 2 (SO 4 ) 3 ⁇ 14H 2 O and ethylene glycol in a weight ratio of 1:2 or with Al 2 (SO 4 ) 3 ⁇ 18H 2 O and ethylene glycol in a weight ratio of 1:2 or Al 2 (SO 4 ) 3 ⁇ 18H 2 O and propylene glycol in a weight ratio of 1:1.6.
• a particulate absorbent polymer by coating a particulate absorbent polymer with an aqueous solution of a polyol which has reacted with the molecular groups close to the surface, preferably with the carboxyl groups, in the presence of a cation of a water-soluble salt while heating in the range from 150 to 250° C., in fact a superabsorbent polymer with a significant improvement in the permeability properties coupled with a very good retention capacity results if the water-soluble salt is present in a certain weight ratio to the polyol and the amount of water added lies within the limits according to the invention.
• the treatment with an aqueous solution of the combination, according to the invention, of after-crosslinking components leads completely unexpectedly to the desired results, that is to say obtaining of polymers with a high retention capacity, even under pressure, with simultaneously excellent permeability properties.
• Successive separate use of the aqueous solution of the organic after-crosslinking agents and the aqueous salt solution with the particular heating does not lead to comparably good product characteristics.
• the required properties also cannot be achieved by a small amount of polyol and large amounts of cations.
• polyols which react with the COOH groups of the polymer close to the surface are employed as the organic after-crosslinking component e) of claim 1 .
• Polyols which are preferably used are aliphatic polyhydroxy compounds with preferably a molecular weight of not more than 250, such as C 2 -C 8 -alkylene diols, such as e.g. ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol and dianhydrosorbitol, C 2 -C 8 -alkylene triols, such as e.g. glycerol and trimethylolpropane, hydroxy compounds of higher functionality, such as e.g. pentaerythritol and sugar alcohols, such as e.g.
• C 2 -C 8 -alkylene diols such as e.g. ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hex
• sorbitol and di- and polyalkylene glycols, such as e.g. diethylene glycol, dipropylene glycol, triethylene glycol, tetraethylene glycol, tetrapropylene glycol, polyethylene glycol and polypropylene glycol, and polyglycols based on 2 or more different alkoxides, such as e.g. a polyglycol of ethylene oxide and propylene oxide units.
• diethylene glycol dipropylene glycol
• triethylene glycol tetraethylene glycol
• tetrapropylene glycol polyethylene glycol and polypropylene glycol
• polyglycols based on 2 or more different alkoxides such as e.g. a polyglycol of ethylene oxide and propylene oxide units.
• the organic after-crosslinking components or mixtures thereof are employed in amounts of 0.01-5 wt. %, preferably 0.1-2.5 wt. %, and more preferably 0.5 to 1.5 wt. %, based on the polymer to be crosslinked.
• the aqueous solutions of water-soluble salts are preferably employed as component f) of claim 1 for crosslinking the carboxylate groups close to the surface.
• the cations of the salts are preferably derived from mono- and polyvalent cations, the monovalent in particular from alkali metals, such as potassium, sodium and lithium, lithium being preferred.
• Divalent cations which are used according to the invention are derived from zinc, beryllium and alkaline earth metals, such as magnesium, calcium and strontium, magnesium being preferred.
• cations of higher valency which can be employed according to the invention are cations of salts of aluminium, iron, chromium, manganese, titanium, zirconium and other transition metals and double salts of such cations or mixtures of the salts mentioned.
• Trivalent cations and cations of higher valency are preferably employed, and of these in particular water-soluble, inorganic salts, and of these aluminium salts and alums and various hydrates thereof, such as e.g. AlCl 3 x6H 2 O, NaAl(SO 4 ) 2 x12H 2 O, KAl(SO) 4 x12H 2 O or Al 2 (SO 4 ) 3 x 14.
• the salt component is employed in amounts, calculated for the cation, of 0.001-1.0 wt. %, preferably 0.005-0.5 wt. %, and preferably 0.01-0.2 wt. %, based on the polymer.
• the preferred weight ratio of water-soluble salt to after-crosslinking agent is preferably 1:1 to 1:3.5, and more preferably 1:1.2 to 1:2.5.
• the water-absorbing polymer which is surface-crosslinked according to the invention is obtained, inter alia, by polymerization of a) 55-99.9 wt. % of a mono-ethylenically unsaturated monomer with acid groups.
• Monomers containing carboxyl groups such as e.g. acrylic acid, methacrylic acid or 2-acrylamido-2-methylpropanesulfonic acid, or mixtures of these monomers are preferred here. It is preferable for at least 50 wt. %, and more preferably at least 75 wt. % of the acid groups to be carboxyl groups.
• the acid groups are neutralized to the extent of at least 25 mol %, i.e. they are present as sodium, potassium or ammonium salts.
• the degree of neutralization is preferably at least 50 mol %.
• Polymers which have been obtained by polymerization of acrylic acid or methacrylic acid, the carboxyl groups of which are neutralized to the extent of 50-80 mol %, in the presence of crosslinking agents are preferred.
• Further monomers b) which can be used for the preparation of the absorbent polymers according to the invention are 0-40 wt. % of ethylenically unsaturated monomers which can be copolymerized with a), such as e.g. acrylamide, methacrylamide, hydroxyethyl acrylate, dimethylaminoalkyl (meth)-acrylate, dimethylaminopropylacrylamide or acrylamidopropyltrimethylammonium chloride. More than 40 wt. % of these monomers can impair the swellability of the polymers.
• a such as e.g. acrylamide, methacrylamide, hydroxyethyl acrylate, dimethylaminoalkyl (meth)-acrylate, dimethylaminopropylacrylamide or acrylamidopropyltrimethylammonium chloride. More than 40 wt. % of these monomers can impair the swellability of the polymers.
• All compounds which carry at least two ethylenically unsaturated double bonds or one ethylenically unsaturated double bond and one functional group which is reactive towards acid groups of the monomers a) or several functional groups which are reactive towards acid groups can be used as the crosslinking component c) which is present during the polymerization of a) and b).
• Examples which may be mentioned are: aliphatic amides, such as e.g.
• methylenebisacryl- or -methacrylamide or ethylenebisacrylamide and furthermore aliphatic esters of polyols or alkoxylated polyols with ethylenically unsaturated acids, such as di(meth)acrylates or tri(meth)acrylates of butanediol or ethylene glycol, polyglycols or trimethylolpropane, di- and triacrylate esters of trimethylolpropane which is preferably oxyalkylated, preferably ethoxylated, with 1 to 30 mol of alkylene oxide, acrylate and methacrylate esters of glycerol and pentaerythritol and of glycerol and pentaerythritol oxyethylated with preferably 1 to 30 mol of ethylene oxide and furthermore allyl compounds, such as allyl (meth)acrylate, alkoxylated allyl (meth)acrylate reacted with preferably 1 to 30
• the absorbent polymers according to the invention can comprise as water-soluble polymers d) 0-30 wt. % of water-soluble polymers, such as partly or completely hydrolysed polyvinyl acetate, polyvinylpyrrolidone, starch or starch derivatives, polyglycols or polyacrylic acids, preferably in polymerized-in form.
• the molecular weight of these polymers is not critical as long as they are water-soluble.
• Preferred water-soluble polymers are starch and polyvinyl alcohol.
• the preferred content of such water-soluble polymers in the absorbent polymer according to the invention is 0-30 wt. %, preferably 0-5 wt. %, based on the total amount of components a) to d).
• the water-soluble polymers, preferably synthetic polymers, such as polyvinyl alcohol can also serve as a graft base for the monomers to be polymerized.
• the usual initiators such as e.g. azo or peroxo compounds, redox systems or UV initiators, (sensitizers), are used for initiation of the free-radical polymerization.
• the polymers according to the invention are preferably prepared by two methods.
• the after-crosslinking components according to the invention are applied in the form of their aqueous solutions.
• Suitable solvents are water and optionally polar, water-miscible organic solvents, which are slightly moist, such as, for example, acetone, methanol, ethanol or 2-propanol or mixtures thereof.
• aqueous solution in the context of the invention means, in respect of the solvent component, that in addition to the water it can also contain organic solvents.
• concentration of the particular after-crosslinking component in the aqueous solvent can vary within wide limits and is in the range from 1 to 80 wt. %, preferably in the range from 5 to 65 wt. %, and more preferably in a range from 10 to 40 wt. %.
• a preferred solution comprises, for example, 1.5-3 parts by wt. of water, 0.5-1 part by wt. of polyol component and 0.4-0.6 part by wt. of an inorganic salt.
• the total amount of solvent is conventionally in the range of 0.5-12 wt. %, preferably 1-7 wt. %, and more preferably 1-5 wt. %, based on the polymer.
• the solution is heated from 20-100° C., preferably from 20-60° C., before application to the polymer.
• a separate, preferably simultaneous metering in of an aqueous solution of the polyol and an aqueous solution of the salt component is also possible if a homogeneous distribution of the two components on the polymer is ensured.
• Application of a single aqueous solution in which the two components are dissolved to the polymer is preferred.
• the after-crosslinking agent solution should be mixed very thoroughly with the polymer particles.
• Suitable mixing units for application of the after-crosslinking agent solution are e.g. Patterson-Kelley mixers, DRAIS turbulence mixers, Lödige mixers, Ruberg mixers, screw mixers, plate mixers and fluidized bed mixers, as well as continuously operating vertical mixers in which the polymer powder is mixed by means of rotating blades at a high frequency (Schugi mixer).
• DRAIS turbulence mixers e.g. Patterson-Kelley mixers, DRAIS turbulence mixers, Lödige mixers, Ruberg mixers, screw mixers, plate mixers and fluidized bed mixers, as well as continuously operating vertical mixers in which the polymer powder is mixed by means of rotating blades at a high frequency (Schugi mixer).
• Schougi mixer continuously operating vertical mixers in which the polymer powder is mixed by means of rotating blades at a high frequency
• the after-crosslinking reaction is carried out at temperatures in the range from at least 150° C. to 250° C., preferably 160° C. to 220° C., and more preferably 170° C. to 200° C.
• the optimum duration of the after-heating can easily be determined for the individual crosslinking agent types with a few experiments. It is limited by when the desired profile of properties of the superabsorber is destroyed again as a result of heat damage.
• the heat treatment can be carried out in conventional dryers or ovens; rotary tube ovens, fluidized bed dryers, plate dryers, paddle dryers or infra-red dryers may be mentioned by way of example.
• agents according to the invention retain the absorbed liquids even under pressure and additionally are capable of distributing further liquid within the construction in the swollen state, they are more preferably employed in higher concentrations, in respect of the hydrophilic fiber material, such as e.g. fluff, than was hitherto possible. They are also suitable for use as a homogeneous superabsorber layer without a fluff content within the nappy construction, as a result of which particularly thin nappies are possible.
• the polymers are furthermore suitable for use in hygiene articles (incontinence products) for adults.
• Such absorbent hygiene products as a rule have a general structure of a liquid-permeable cover (1) facing the body, a liquid-absorbing absorbent layer (2) and a substantially liquid-impermeable outer layer (3) facing away from the body. Further constructions are optionally also used for rapid absorption and distribution of body fluid (4) in the absorbent core. These constructions are often, but not necessarily, employed between the liquid-permeable cover (1) facing the body and the liquid-absorbing absorbent layer (2).
• the liquid-permeable cover (1) as a rule comprises a nonwoven, fibrous fleece or another porous construction.
• Possible materials for this cover (1) are e.g. synthetic polymers, such as, for example, polyvinyl chloride or fluoride, polytetrafluoroethylene (PTFE), polyvinyl alcohols and derivatives, polyacrylates, polyamides, polyesters, polyurethanes, polystyrene, polysiloxanes or polyolefins (e.g. polyethylene (PE) or polypropylene (PP)) and naturally occurring fiber materials, as well as any desired combinations of the abovementioned materials in the sense of blended materials or composite materials or copolymers.
• synthetic polymers such as, for example, polyvinyl chloride or fluoride, polytetrafluoroethylene (PTFE), polyvinyl alcohols and derivatives, polyacrylates, polyamides, polyesters, polyurethanes, polystyrene, polysiloxanes or polyolef
• the liquid-permeable cover (1) has a hydrophilic character. It can furthermore comprise a combination of hydrophilic and hydrophobic constituents.
• a hydrophobic treatment of the liquid-permeable cover (1) is as a rule preferred, in order to allow rapid seepage times of body fluid into the liquid-absorbing absorbent layer (2), but partly hydrophobized covers (1) are also used.
• the liquid-absorbing absorbent layer (2) comprises the superabsorbent powder or granules and optionally further components of, for example, fibrous materials, foamed materials, film-forming materials or porous materials, as well as combinations of two or more of these materials.
• fibrous materials for example, fibrous materials, foamed materials, film-forming materials or porous materials, as well as combinations of two or more of these materials.
• Each of these materials can be of either natural or synthetic origin, or can have been prepared by chemical or physical modification of naturally occurring materials.
• the materials can be hydrophilic or hydrophobic, hydrophilic materials being preferred. This applies in particular to those compositions which are to efficiently absorb the body fluids secreted and transport them in the direction of regions of the absorbent core further removed from the entry point of the body fluid.
• Suitable hydrophilic fiber materials are e.g. cellulose fibers, modified cellulose fibers (e.g. reinforced cellulose fibers), polyester fiber (e.g. Dacron), hydrophilic nylon and also hydrophilized hydrophobic fibers, such as e.g. surfactant-hydrophilized polyolefins (PE, PP), polyesters, polyacrylates, polyamides, polystyrene, polyurethanes and others.
• PE surfactant-hydrophilized polyolefins
• Cellulose fibers and modified cellulose fibers are preferably employed.
• Combinations of cellulose fibers and/or modified cellulose fibers with synthetic fibers, such as e.g. PE/PP composite materials, so-called bi-component fibers, such as are used e.g. for thermobonding of airlaid materials, or other materials are also customary.
• the fiber materials can be in various use forms, e.g. as loose cellulose fibers separated out or laid out of an air stream or an aqueous phase, as nonwoven fleece or as tissue. Combinations of various use forms are possible.
• pulverulent substances can optionally be employed in addition to the superabsorbent polymers according to the invention, such as e.g. odour-binding substances, such as cyclodextrins, zeolites, inorganic or organic salts and similar materials.
• odour-binding substances such as cyclodextrins, zeolites, inorganic or organic salts and similar materials.
• Polymer foams such as are described in the specifications DE 44 18 319 A1 and DE 195 05 709 A1 can be employed e.g. as porous materials and foamed materials.
• Thermoplastic fibers e.g. bi-component fibers of polyolefins
• polyolefin granules e.g., polyolefin granules
• latex dispersions or hot-melt adhesives can be used for mechanical stabilization of the liquid-absorbing absorbent layer (2).
• One or more layers of tissue are optionally used for stabilization.
• the liquid-absorbing absorbent layer (2) can be one layer or can comprise several layers. Constructions which comprise hydrophilic fibers, preferably cellulose fibers, optionally of a construction for rapid absorption and distribution of body fluid (4), such as, for example, chemically reinforced (modified) cellulose fibers or high loft fleeces of hydrophilic or hydrophilized fibers, and superabsorbent polymers can be used for this.
• the superabsorbent polymers according to the invention can be distributed here homogeneously in the cellulose fibers or the reinforced cellulose fibers, but they can also be introduced as a layer between the cellulose fibers or the reinforced cellulose fibers, or the concentration of the superabsorbent polymers can have a gradient within the cellulose fibers or reinforced cellulose fibers.
• the ratio of the total amount of superabsorbent polymer and the total amount of cellulose fibers or reinforced cellulose fibers in the absorbing absorbent core can vary between 0-100 wt. %, in one embodiment concentrations of up to 100% of superabsorbent polymers being possible locally, e.g. with introduction of a gradient or introduction in layers.
• the various superabsorbers can be introduced into the absorbent cushion as a mixture with one another, or they can be placed in different locations in the absorbent core. Such a different placing can take place in the direction of the thickness of the absorbent cushion or of the length or width of the absorbent cushion.
• the liquid-absorbing absorbent layer (2) comprises one or more of the layers comprising superabsorbent polymers according to the invention, optionally with cellulose fibers or reinforced cellulose fibers.
• constructions of combinations of layers with homogeneous introduction of the superabsorber and additionally introduction in layers are used.
• the absorption articles can optionally have further layers of pure cellulose fibers or reinforced cellulose fibers on the side facing the body and/or also the side facing away from the body.
• the entire absorbent cushion or also individual layers of the liquid-absorbing absorbent layer (2) can optionally be separated by layers of tissue of other components of the absorption article or are in direct contact with other layers or components.
• the construction for rapid absorption and distribution of body fluid (4) and the liquid-absorbing absorbent layer (2) can be separated from one another by tissue or can be in direct contact with one another. If no separate construction for rapid absorption and distribution of body fluid (4) exists between the liquid-absorbing absorbent layer (2) and the liquid-permeable cover (1) facing the body, but the effect of liquid distribution is to be achieved e.g. by the use of a specific liquid-permeable cover (1) facing the body, the liquid-absorbing absorbent layer (2) can also optionally be separated from the liquid-permeable cover (1) facing the body by a tissue.
• nonwoven fleece can optionally also be introduced into the liquid-absorbing absorbent layer (2). Both components lead to the desired secondary effect of stabilization and strengthening of the absorption core in the moist state.
• liquid-absorbing absorbent layers in particular fiber-containing liquid-distributing and -storing layers containing superabsorbent polymers, can be prepared by a diversity of preparation processes.
• Chemically reinforced, modified cellulose fibers can be produced, for example, from cellulose fibers which are converted in a chemical reaction by crosslinking agents, such as e.g. C 2 -C 8 dialdehydes, C 2 -C 8 monoaldehydes with an additional acid function or C 2 -C 9 polycarboxylic acids.
• crosslinking agents such as e.g. C 2 -C 8 dialdehydes, C 2 -C 8 monoaldehydes with an additional acid function or C 2 -C 9 polycarboxylic acids.
• Specific examples are: glutaraldehyde, glyoxal, glyoxalic acid or citric acid.
• Cationically modified starch or polyamide-epichlorohydrin resins e.g. KYMENE 557H, Hercules Inc., Wilmington, Del.
• a twisted, crimped structure which has an advantageous effect on the rate of absorption of liquid is achieved and stabilized by the crosslinking.
• the absorbent hygiene products can vary widely in their weight per unit area and thickness and therefore density.
• the densities of the regions of the absorption cores are typically between 0.08 and 0.25 g/cm 3 .
• the weights per unit area are between 10 and 1,000 g/m 2 , weights per unit area of between 100 and 600 g/m 2 preferably being realized (see also U.S. Pat. No. 5,669,894).
• the density varies over the length of the absorbent core. This occurs as a consequence of a controlled metering of the amount of cellulose fibers or reinforced cellulose fibers or of the amount of the superabsorbent polymer, since in preferred embodiments these components are introduced to a higher degree into the front region of the absorbent disposable article.
• the polymers according to the invention are also employed in absorber articles which are suitable for further uses. For this, they are processed to a web by mixing with paper or fluff or synthetic fibers or by distributing the superabsorbent polymers between substrates of paper, fluff or non-woven textiles or by processing into carrier materials.
• the polymers according to the invention are furthermore also used in all instances where aqueous liquids must be absorbed, such as e.g. in cable sheathings, in foodstuffs packaging, in the agricultural sector in plant growing and as a water store and as an active compound carrier with a time-delayed release of the active compound to the environment.
• the superabsorbent polymers according to the invention show a significant improvement in permeability, i.e. an improvement in the transportation of liquid in the swollen state.
• Polymers with permeability values (SFC) of up to 70 ⁇ 10 ⁇ 7 cm 3 sec/g at a retention (TB) of at least 27 g/g are obtained, preferably polymers with SFC values of >70 ⁇ 10 ⁇ 7 to ⁇ 150 ⁇ 10 ⁇ 7 cm 3 sec/g at a retention (TB) of at least 25 g/g.
• the polymers according to the invention show measurement values for the absorption of liquid under pressure (AAP 0.7 psi) of at least 18 g/g.
• the retention (TB), absorption under pressure (AAP) and permeability to 0.9% sodium chloride solution in the swollen state (SFC) are determined.
• the absorption under pressure (pressure load 50 g/cm 2 ) is determined by a method described in EP 0339461, page 7. Approx. 0.9 g superabsorber is weighed into a cylinder with a perforated base. The superabsorber layer uniformly sprinkled on is loaded with a plunger which exerts a pressure of 50 g/cm 2 . The previously weighed cylinder is then placed on a glass filter plate in a dish containing 0.9% NaCl solution, in which the level of liquid corresponds exactly to the height of the filter plate. After the cylinder unit has been allowed to soak up 0.9% NaCl solution for 1 hour, this is re-weighed and the AAP is calculated as follows:
• AAP final weight (cylinder unit+superabsorber)-amount weighed out (cylinder unit+completely soaked superabsorber)/amount of superabsorber weighed out.
• 0.118 M NaCl solution is allowed to run from a levelled reservoir vessel through the swollen gel layer under a constant hydrostatic pressure.
• the swollen gel layer is covered with a special perforated cylinder which ensures a uniform distribution of the 0.118 M NaCl solution above the gel and constant conditions (measurement temperature 20-25° C.) during the measurement in respect of the nature of the gel bed.
• the pressure acting on the swollen superabsorber continues to be 20 g/cm 2 .
• the amount of liquid which passes through the gel layer as a function of time is recorded at intervals of 20 seconds within a period of time of 10 minutes with the aid of a computer and a balance.
• Formal addition of the numerical values of the tea-bag retention and the SFC value illustrates the sudden increase in this combination of properties in polymers according to the invention compared with untreated superabsorber powders or products which have been after-crosslinked on the surface by known methods.
• the numerical value is not achieved by a high contribution of one of the two values (e.g. a high TB retention value and a low SFC value and vice versa) in the products according to the invention.
• 50 g powder A were mixed with vigorous stirring with a solution of 0.5 g 1,3-propanediol, 1.25 g water and 0.25 g aluminium sulfate 18-hydrate and the mixture was then heated for 30 min in an oven which was temperature-controlled at 180° C.
• 50 g powder A were mixed with a solution of 0.5 g 1.3-propanediol and 1.25 g water and the mixture was then heated for 30 min in an oven which was temperature-controlled at 130° C.
• 0.84 g triallylamine and 1.5 g polyethylene glycol (750) monoallyl ether-acrylate, as the crosslinking agent, are dissolved in 965.175 g of an aqueous solution of sodium acrylate with a degree of neutralization of 70 mol% (monomer concentration: 37.7%).
• the monomer solution is flushed through with nitrogen in a polymerization vessel of plastic for 30 minutes in order to remove the dissolved oxygen.
• the polymerization is started at a temperature of 4° C.

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